1. Field of the Invention
The present invention relates to a process for producing terephthalic acid. Particularly, it relates to a process for producing terephthalic acid which is suitable for use as a starting material for the production of a polyester such as polyethylene terephthalate.
2. Discussion of Background
Crude terephthalic acid obtained by oxidation of p-xylene usually contains relatively large amounts of various impurities including 4-carboxybenzaldehyde (hereinafter referred to simply as "4CBA"). Heretofore, it has been common that such terephthalic acid is purified and then used as a starting material for a polyester. As a purification method for such crude terephthalic acid, a method is common wherein crude terephthalic acid is dissolved in an aqueous medium and contacted with a platinum group metal catalyst at a high temperature under a high pressure for purification (U.S. Pat. No. 3,584,039).
Further, as a method for recovering terephthalic acid crystals after the purification reaction, a method is known wherein an aqueous solution or an aqueous slurry of terephthalic acid is cooled for crystallization stepwise in a plurality of crystallizers connected in series. For example, U.S. Pat. No. 3,931,305 discloses a method wherein cooling is carried out by setting the temperature of the crystallizer so that the precipitation rate of crystals will be smaller than in the previous step, to prevent inclusion into terephthalic acid of a p-toluic acid formed by hydrogenation of 4-carboxybenzaldehyde and thereby to maintain the purity of terephthalic acid. In this publication, there is no disclosure with respect to the relation between the various crystallization conditions and the mean particle diameter or particle size distribution of terephthalic acid crystals.
On the other hand, in recent years, it has become common to produce a polyester, particularly polyethylene terephthalate, by a so-called direct polymerization method, wherein terephthalic acid and a glycol are directly reacted. In this direct polymerization method, terephthalic acid is mixed with a glycol such as ethylene glycol to form a slurry, which is then sent to a reaction system and subjected to the reaction. In such a case, in order to increase the uniformity of the reaction, the fluidity of the slurry of terephthalic acid is desired to be excellent. Further, a good powder flowability is also required for the handling of the powder such as transportation or storage of terephthalic acid.
The fluidity of the slurry of terephthalic acid is substantially influenced by the particle size distribution or the mean particle diameter of terephthalic acid particles. Usually, the slurry properties are improved when the particle size distribution is wide with particles having large to small particle sizes, and the mean particle diameter is usually within a range of from 50 to 150 .mu.m. If the proportion of particles having large particle sizes exceeding 200 .mu.m increases too much, terephthalic acid tends to remain unreacted at the time of the direct polymerization, and consequently, it will be necessary to take a long reaction time, and there will be a problem such that a by-product increases.
In order to attain the uniformity of the reaction and excellent slurry properties, a large amount of a glycol may be employed relative to terephthalic acid. However, an excessive amount of glycol tends to increase a side reaction at the time of the polycondensation reaction and causes a decrease in the polymerization degree and the melting point of the polymer and even discoloration. To avoid such drawbacks, the amount of the glycol may be brought to a level as close as possible to the stoichiometric amount. However, if the amount of the glycol is decreased, it will be necessary to increase the required power for stirring the reactor and the tank for preparation of the slurry. Further, the fluidity and the reactivity of the slurry tend to be poor, and consequently there will be a problem such that the required reaction time will be long. Accordingly, terephthalic acid particles capable of forming a slurry having good fluidity and reactivity with a required minimum amount of the glycol, is most suitable as a starting material for the direct polymerization.
As a method for producing such terephthalic acid, Japanese Unexamined Patent Publication No. 29735/1973 discloses a method wherein terephthalic acid particles having a mean particle diameter of at least 100 .mu.m and terephthalic acid particles having a mean particle diameter of at least 50 .mu.m are mixed in a ratio of 70-85% to 30-15%. However, this method requires at least two series of crystallizing steps and is uneconomical since it requires an installation for separately storing the produced terephthalic acids, followed by mixing them again. On the other hand, Japanese Unexamined Patent Publication No. 20303/1974 discloses a method wherein recycling stirring treatment is conducted by a pump in a state of a slurry suspended in a solvent having a small solubility, so that the particle size is slightly reduced, while the apparent density is gradually improved to obtain terephthalic acid having a good slurry property. According to this method, terephthalic acid having a good slurry property can be obtained, but since the particle size is reduced, the flowability as the powder tends to be impaired, such being undesirable from the viewpoint of the handling efficiency of the powder.
Further, when multi-stage crystallization is conducted by a plurality of crystallizers, the higher the temperature in the first crystallizer is, the more the recovery efficiency of thermal energy can be improved. However, from the viewpoint of a person skilled in the art, there will be a problem that if the crystallization temperature is too high, the particle size of crystallized terephthalic acid particles tends to be too large.